Polyarylene sulfide based resins (hereinafter referred as a “PAS based resin”), particularly polyphenylene sulfide based resins (hereinafter referred as a “PPS based resin”) out of them are known as engineering plastics which are excellent in a mechanical strength, a heat resistance, a flame retardancy and a solvent resistance and which have good electrical characteristics and a high rigidity, and they are widely used as various materials such as materials for electronic and electric equipment parts and the like.
A process in which a dihalogenated aromatic compound such as p-dichlorobenzene is reacted with a sodium salt such as sodium sulfide in an aprotic organic solvent such as N-methyl-2-pyrrolidone (hereinafter referred as the “NMP”) has so far usually been used for producing the above resins. In the above process, however, sodium halide is by-produced, and this sodium halide is introduced into the resin since it is insoluble in the solvent such as NMP and the like. Accordingly, it has been impossible to sufficiently remove sodium halide contained in a PAS based resin even when the resin is washed with a large amount of water after polymerization.
Accordingly, attentions have been paid to polymerization carried out using a lithium salt in place of a sodium salt. Lithium halide by-produced during polymerization is soluble in a lot of aprotic organic solvents (solvents for polymerization) such as NMP, and therefore it becomes possible to relatively readily reduce a concentration of lithium contained in the resin. However, by-produced lithium halide remains in a PAS based resin as an impurity, and it has been impossible to sufficiently remove it even when washed with a large amount of water after polymerization.
A method in which a polyarylene sulfide based resin staying in a softened or molten state is washed with a solvent such as an organic amide-water mixed solvent and the like is proposed as a method for effectively removing alkali metal halides by-produced in producing a polyarylene sulfide resin (refer to, for example, Japanese Unexamined Patent Application Laid-Open No. 228023/1986 and Japanese Unexamined Patent Application Laid-Open No. 207027/1995). In the above washing methods, however, although alkali metal halides can effectively be removed, a part of the polyarylene sulfide resin is dissolved in the liquid phase together with the alkali metal halides, and therefore the problem that the polyarylene sulfide resin obtained is reduced in a yield has been involved therein.
The present inventors have proposed a method in which a solvent saturated with a PAS based resin is used as a washing liquid for the purpose of overcoming the above problem (refer to Japanese Unexamined Patent Application Laid-Open No. 273174/2000). However, when the above method is applied to an actual production process, involved therein are the problems that it is difficult to control a flow amount of a washing liquid in a washing step and that pipelines in a production apparatus are complicated. Further, according to the above method, a loss in a PAS based resin is reduced, but there has been the problem that a part of the PAS based resin is dissolved in the washing liquid and lost together with the washing liquid, so that the satisfactory yield is not necessarily obtained.